Nano-characterization of ceramic top-Coat/metallic bond-coat interface for thermal barrier coating systems by plasma spraying

Characterization at the ceramic top-coat/metallic bond-coat interfacial region was conducted for several kinds of the plasma sprayed thermal barrier coating (TBC) systems by means of a transmission electron microscope (TEM), an electron probe micro-analysis (EPMA) and so forth, in order to find out the optimum compositional and structural conditions of the coating components together with the optimum coating processing condition for designing the advanced TBC systems. Specimens with different coating features were prepared systematically by using different coating parameters such as the top-coat spraying conditions and reheat-treatment conditions. Especially, the reheat-treatment was applied to the TBC specimen with different temperature either in air or in inert argon (Ar) gas atmosphere. It was found that in the case of reheat-treatment in air the thermally grown oxide (TGO) was developed at the interface as multiple oxide layers; one is Al2O3 layer developed discontinuously at directly above the bond-coat and another is the mixed oxides layer consisting of the Al, Cr, Co, Ni oxide particles on the Al2O3 layer. Such a TGO layer was heterogeneous and imperfect layer with containing many kinds of defects. On the contrary, the TGO layer formed by the reheat-treatment in Ar was composed dominantly of the continuous and fairly purified Al2O3 layer with large grain size and homogeneous layer thickness. The growth mechanism and influencing factors for TGO were discussed in some detail on the basis of the nano-characterization and quantitative evaluation of TGO.